The present invention relates generally to induction motors, and more particularly to a method and apparatus to reduce starting currents in a cage induction motor.
Induction motors are used in a wide variety of consumer and industrial applications, and generate rotor torque via stator currents that magnetic fields rotating at a synchronous speed ns. These changing magnetic fields induce opposing rotor currents according to Lenz's law, and drive rotors to an operating rotor speed n<ns. The difference between synchronous speed ns and rotor speed n is characterized by slip s, where:
                    s        =                                            n              s                        -            n                                n            s                                              [                  Equation          ⁢                                          ⁢          1                ]            
During normal sustained operation, slip is low (e.g. s=˜0.05), and stator current is not significantly affected by slip. At the beginning of motor startup, however, slip is high (s=1 when stationary, and close to 1 immediately thereafter), and can cause dramatic increases in stator current over optimal operating levels. In some cases, stator currents during startup can be 5-10 times greater than corresponding currents during sustained operation. High startup currents are particularly problematic in large motors, where excessive currents can damage motor components or otherwise reduce part lifetimes. A variety of methods are commonly used to reduce stator current during startup, mostly through the use of peripheral power electronics such as solid state soft starters, Some methods include Y-Delta switches, skin effects produced by deep-bar or double-cage rotors, and autotransformers. In general, many induction motors include some sort of system or device to reduce starting rotor currents to non-damaging levels.